In Lorain as elsewhere, explanations for marital decline start with home economics: men are worth less they used to be. Among men with some college but no degrees, earnings have fallen 8 percent in the past 30 years, according to the Bureau of Labor Statistics, while the earnings of their female counterparts have risen by 8 percent.

“Women used to rely on men, but we don’t need to anymore,” said Teresa Fragoso, 25, a single mother in Lorain. “We support ourselves. We support our kids.”

--For Women Under 30, Most Births...